Although Isaac belonged to the non-Chalcedonian Church of the East, his writings--originally in Syriac--deal almost exclusively with prayer and other ascetical matters and avoid divisive Christological issues. He has therefore been highly regarded--and honored as a saint--by the Orthodox, Monophysite, and Roman Catholic churches. A distinctive feature of his writings are his notions of hell and eternal punishment. Basically, for him hell exists but is not forever; it is God's version of "tough love", a necessary stage of purification on the way to resurrection and eternal life. God's love for all creatures is so great that even demons can be saved eventually. A good discussion of Isaac's view of salvation contains lengthy quotes from his writings which are excerpted below.
For it would be most odious and utterly blasphemous to think that hate or resentment exists with God, even against demonic beings...Rather, He acts towards us in ways He knows will be advantageous to us, whether by way of things that cause suffering, or by way of things that cause relief...
...with Him it is not a matter of retribution, but He is always looking beyond to the advantage that will come from His dealing with humanity. And one such thing is this matter of Gehenna.
It is not the way of the compassionate Maker to create rational beings in order to deliver them over mercilessly to unending affliction in punishment for things of which He knew even before they were fashioned, aware how they would turn out when He created them--and whom nonetheless He created.
I also maintain that those who are punished in Gehenna are scourged by the scourge of love. For what is so bitter and vehement as the punishment of love? I mean that those who have become conscious that they have sinned against love suffer greater torment from this than from any fear of punishment...Thus I say that this is the torment of Gehenna: bitter regret.
...The wicked, who all their life have turned aside to evil deeds, after they have been set in order in their minds by punishments and the fear of them, choose the good, having come to learn how much they have sinned...and so eventually they are held worthy of the felicity of divine munificence. For Christ would never have said "Until you pay the last farthing" unless it had been possible for us to be freed from our sins once we had recompensed for them through punishments.